[MukundaKatta]

Summary

Refs #58. Expands test coverage with edge cases and identity checks. No library code touched.

Coverage delta

numpy_financial/_financial.py: 97% (6 missing) → 99% (2 missing). The 2 remaining lines are unreachable defensive branches.

What's added

83 new test cases in 8 classes covering:

• Edge cases: zero-rate paths for fv/pmt/pv/nper, infinite cashflows, scalar/0-d/array shapes, negative rates
• Error paths: npv invalid shape, mirr size mismatch (both raise + nan-return), irr no-real-solution, rate iterations-exceeded array path
• Vectorized vs scalar parity for fv, pmt, pv
• Analytical round-trips: pmt = ppmt + ipmt, npv(irr(x), x) = 0, fv/pv inverse relations
• _convert_when alias coverage (9 aliases + ndarray pass-through + KeyError)
• _irr_default_selection mixed-sign / same-sign / all-negative branches + custom selection_logic injection

Files changed

• numpy_financial/tests/test_coverage_expansion.py (new, 550 lines, 83 tests)

Test plan

• All 193 tests pass (110 existing + 83 new)
• ruff format and ruff check clean on the new file

[milliondreamsblog]

One policy question raised by test_irr_default_selection_mixed_signs_picks_positive: for mixed-sign candidates, pos is positive and neg is negative, so pos >= neg always holds. The test comment describes that comparison as a condition, but the current behaviour is simply to prefer a non-negative root before choosing the smallest magnitude.

A cash-flow example makes the distinction visible:

import numpy_financial as npf
npf.irr([-100, 207, -106.4])
# 0.11999999999999855

Both -5% and 12% make NPV zero here. The returned 12% is valid, but it is not the root closest to zero.

Is that preference intended? If so, could the test comment and the selection-policy documentation state it directly? The implementation comment also mentions comparing cash flows, although _irr_default_selection receives candidate rates.

I checked this against 1.1.0 and the financial module at this PR's head, aeff04c. They agree. This is a policy/documentation question, rather than a request to force coverage of the unreachable branch.